This invention relates generally to current limiting means for protecting electrical apparatus and is concerned more particularly with a vacuum fuse for interrupting an electrical circuit in response to a current overload.
Components of an electrical circuit usually are protected from damage due to current overloads by a suitable type of circuit interrupting device, such as a vacuum fuse, for example. A vacuum fuse generally comprises an evacuated envelope having therein a fusible element electrically connected to a supporting pair of mutually spaced electrodes which are attached to respective external terminals of the fuse. The fusible element is provided with a much smaller cross-section than the support electrodes to ensure a rapid response when excessive current flows through the fuse.
In operation, the vacuum fuse may be series connected, for example, in an alternating current circuit such that the current flowing through the circuit passes through the fuse. Consequently, when the current exceeds a predetermined value, the fusible element melts and vaporizes thereby interrupting the circuit. The resulting gap between the support electrodes may be bridged by a current arc which is extinguished when the half cycle of the alternating current approaches zero value. During the next half cycle, the arc will not be re-ignited provided that a high dielectric is maintained in the gap between the support electrodes. Thus, after vaporization of the fusible element, it is required that the dielectric characteristics of the vacuum remain sufficiently high to prevent voltage breakdown across the gap between the support electrodes.
Accordingly, the effectiveness of a vacuum fuse depends not only on the response accuracy of the fusible element, but also on the dielectric strength of the vacuum within the envelope. Consequently, during processing, the component parts of the tube preferably are outgassed before assembly. Also, it is desirable that the component parts be baked out while the envelope is being evacuated in order to remove substantially all of the occluded gases before sealing the envelope. However, the relatively bulky support electrodes generally are more difficult to outgas than the other component parts of the fuse. Furthermore, the reduced cross-section of the fusible element presents handling problems during assembly and outgassing of the fuse.
Therefore, it is advantageous and desirable to provide a vacuum fuse with a simple and rugged structure which facilitates handling and outgassing of the component parts of the fuse during production thereof.